Every ball rolled changes the oil pattern in one way or the other.  Polyester and rubber balls are likely moving the oil to the back ends, while the reactives are absorbing some with each shot. 

Add to that the varying lane surfaces and lane machines and it's no wonder why things change.  We bowl on a mostly "invisible" and changing condition all the time.  That's the part of the sport that many of us enjoy.  If I knew before I arrived that I'd be sliding on ___ board and targeting at ___ board and the reaction was the same each time and all night I'd lose interest fast.  Embrace the challenge of the game and find enjoyment in the process, and you'll be hooked for life.